1. Field of the Invention
The invention relates to fiber optical circulators, and particularly to the circulator with a simplified structure thereof.
2. The Related Art
An optical circulator is a passive, non-reciprocal device that is widely used in fiber optical communication systems and subsystems such as bi-directional transmission systems and optical add/drop multiplexers. A circulator should have at least three-ports, where the light entering from Port 1 goes to Port 2 while the light entering from Port 2 goes to Port 3. To meeting the ever-demanding requirements of higher performance, better reliability, easier fabrication and lower cost, people have been seeking simpler and more miniaturized designs.
Most of the traditional optical circulator designs normally have a collimator device such as a GRIN lens (Graded index lenses) to couple light into or from the fiber of each port. For a three-port circulator, three collimating lenses would have to be used in those designs. As a result, the traditional circulators are often very bulky and expensive since the price goes up quickly as the sizes of crystals increase. At the same time, the alignment and reliability can be troublesome if there are many optical elements used in the circulators.
As the optical communication technology becomes more mature and more widely deployed, the need to shrink the optical circulator size and to decrease the cost must be met. Several designs have been proposed to make the linear shaped circulators which consist of only two collimating lenses to enable smaller package sizes. In those designs, Port 1 and Port 3 share the same one collimating lens with each other. To do this, the forward light (from Port 1 to Port 2) must leave the collimator of Port 1 and 3 with a different beam angle from that of the backward light (from Port 2 to Port 3) which enters the same collimator. To create the angle difference, several mechanisms were proposed in the prior arts. Generally, those techniques were either difficult to realize since many elements are requisitely involved to create this angle difference, and/or the solutions were too expensive. U.S. Pat. Nos. 4,272,159, 4,650,289, 5,204,771, 5,400,418, 5,471,340, 5,574,596, 5,682,446, 5,689,593, 5,818,981, 5,909,310, 5,923,472, 6,002,512, 6,014,475, 6,026,202, 6,052,228, 6,075,596, 6,088,491, 6,111,695 and 6,178,044, show some approaches.
Therefore, an object of the invention is to provide a circulator with a simplified structure, thus resulting in robustness, good performance and the low cost thereof.